Muslim pilgrims circle the Kaaba and pray inside the Grand Mosque on Oct. 23 in Mecca, Saudi Arabia. Three million Muslims conduct a pilgrimage, or Hajj, to the city each year, which makes this event the largest yearly gathering of people in the world. Hassan Ammar, AP

Muslim pilgrims pray at the top of Noor Mountain near the Hiraa Cave on Oct. 21. Hassan Ammar, AP

Pilgrims pray on the summit of Noor Mountain near the Hiraa Cave. Muslims believe the prophet Mohammed received his first divine revelation in the cave. Hassan Ammar, AP

Pilgrims circle the Kaaba. The Kaaba, a structure wrapped with a black curtain, is Islam's holiest building and measures 60 feet high by 60 feet in length. Hassan Ammar, AP

Thousands of Muslim pilgrims pray around the Kaaba. Inside the Kaaba, in the eastern corner of the building, is an artifact set in gold called the Black Stone. Muslims believe this stone was kissed by the Prophet Mohammed. Hassan Ammar, AP